1. Field of the Invention
This invention relates to an improvement relating to a cutting tool material using, as a substrate, a sintered compact comprising a high pressure phase boron nitride such as cubic boron nitride (cBN), wurtzite boron nitride (wBN), etc. which will hereinafter be referred to as xe2x80x9ccBN sintered compactxe2x80x9d, and in particular, it is concerned with a cutting tool of a surface-coated boron nitride having a hard coated film excellent in adhesive strength as well as smoothness.
2. Description of the Related Art
cBN (cubic boron nitride) has a hardness and thermal conductivity next to diamond and excellent characteristics such that reactivity with ferrous metals is lower as compared with diamond. Thus, a cutting tool using a cBN sintered compact containing this cBN has displaced grinding or working tools and cutting tools of cemented carbides or cermets in working hard-to-cut ferrous materials from the standpoint of improving the working efficiency and reducing the installation cost.
The cBN sintered compact for a cutting tool is a sintered compact comprising cBN particles and a binder, which can generally be classified into the following two types:
(1) Sintered compact well-balanced in wear resistance as well as strength and mainly used for hardened steels, which comprises 30 to 80 volume % of cBN particles, the cBN particles being bonded through a binder predominantly consisting of Ti type ceramics such as TiN, TiC, TiCN, etc.
(2) Sintered compact excellent in thermal conductivity as well as strength and mainly used for cast irons, which comprises 80 to 90 volume % of cBN particles, the cBN particles being directly bonded and the balance of a binder predominantly consisting of an Al compound or Co compound.
cBN compacts for cutting tools have lately been watched with keen interest as a tool adaptable for the global environment problem, since consciousness for the global environment problem is more increasing, for example, as to reduction of industrial wastes such as grinding sludges and decrease of consumed electric power as a countermeasure for the global warming problem.
However, cBN particles have disadvantages that affinity to ferrous metals is larger than TiN, TiC, TiCN binders, in addition to having cleavage property, in spite of that the cBN particles have a higher hardness as represented by a Vickers hardness Hv of about 5,000 (the hardness Hv of the binder being at most about 2,000 to 2,800). Accordingly, further life lengthening is desired for the cBN compact tool because of finally expiring due to thermal abrasion and breakage of the tool edge due to progress of abrasion.
In particular, of late, the expectations for the cBN compact tool have similarly increased ever more from the standpoint of diversification of hardened steel parts and increasing functionality, and it has been desired to develop a tool capable of working a surface of a workpiece in a high grade and with a high speed and high efficiency.
For example, in order to further improve the wear resistance and fracture strength of a cBN sintered compact, a method has been proposed comprising coating the cBN sintered compact with a film of TiN, TiCN, TiAlN, etc.
However, a cBN sintered compact tool meets with a problem that an unexpected delamination of a coated film occurs often, resulting in dispersion of the service life because of being used under severe conditions.
For the purpose of improving the adhesive strength of a coated film, there has been proposed a tool, which is used for previously roughening a surface of substrate in a roughness of 0.5 to 12.5 xcexcm and film making on the roughened surface, for example, as shown in JP-A-7-18415 or U.S. Pat. No. 5,709,907. In the case of carrying out cutting using these tools, the shape of the tool surface is transferred to a workpiece and accordingly, the dimensional precision and grade of the worked surface is worsened as compared with the prior art cBN sintered compacts. Since the cBN tool is often applied to uses requiring a working precision with xcexcm order, in particular, practical cutting by these tools is difficult.
On the other hand, as a means for improving the adhesive strength of a coated film without surface-roughening a cBN substrate, there has been proposed a tool comprising a cBN sintered compact whose surface is coated with a hard coated film consisting of titanium nitride through a metallic Ti layer with an average layer thickness of 0.05 to 0.3 xcexcm, for example, as shown in JP-A-1-96083 or JP-A-1-96084. According to these methods, however, a cutting edge is exposed to a high pressure and high temperature environment during cutting a relatively hard, hard-to-cut ferrous material and accordingly, there arises a problem that metallic titanium thus softens and the hard coated film is easily broken and delaminated.
In a TiAlN-coated cBN tool of JP-A-8-119774, there is disclosed such a technique that a TiN layer is coated as an intermediate layer between a cBN substrate and TiAlN-coated film to bond the TiAlN-coated film thereto with a high adhesive strength without surface roughening the cBN substrate, but according to this technique, further improvement of the adhesive strength between the cBN substrate and TiN film cannot be expected in principle and in the case of coating a cBN sintered compact with a small content of TiN or TiC, that is, high cBN content sintered compact, moreover, lowering of the adhesive strength of the hard coated film is unavoidable.
Accordingly, the inventors have made studies on the mechanism of breakage or delamination of the hard coated film in the coated cBN sintered compact tool for the purpose of developing a tool capable of providing a high grade worked surface maintained for a long time without delaminating the hard coated film in the coated cBN sintered compact tool even if the tool is subjected to cutting under severer conditions.
Consequently, the inventors have found that:
(I) a cBN sintered compact is bonded with a hard thin film such as TiN, TiCN or TiAlN film by diffusion or reaction between the substrate and the hard coated film, during which a stronger adhesion occurs to the binder part rather than to the hardly sinterable cBN particles to maintain the adhesive strength of the hard coated film and substrate and
(II) When an excessive stress remains in the hard coated layer or when an impact is loaded on a hard coated film from outside, for example, during working a workpiece having an interrupted part, film delamination due to breakage in the interface between the hard coated film and cBN sintered compact or in the hard coated film takes place by such a stress.
It is an object of the present invention to provide an improved cutting tool based on a sintered compact comprising a high pressure phase type boron nitride such as cBN, wBN, etc., having a hard coated film excellent in adhesive strength as well as smoothness.
It is another object of the present invention to provide a surface-coated high pressure phase boron nitride sintered compact tool in which the adhesive strength of the hard coated film to the substrate is remarkably improved, in particular, noticing an improvement of an intermediate layer, whereby the problems of the prior art, as described above, can be solved.
These objects can be accomplished by a surface-coated boron nitride sintered compact tool, in which at least a part of the tool edge related with cutting is based on a substrate of a high pressure phase type boron nitride, and a surface-coated layer is allowed to be present on the surface thereof, the said surface-coated layer comprising an intermediate layer present in continuous manner to the interface of the said high pressure phase boron nitride sintered compact substrate and a hard coated layer formed further thereon, the said intermediate layer consisting of at least one element selected from the Group 4a, 5a and 6a of Periodic Table having a mean thickness of 0.005 to 0.05 xcexcm, preferably 0.02 to 0.04 xcexcm.